The present invention relates to a coupling member forming the top guide in a three point linkage coupling used for attaching various kinds of agricultural implements to farm tractors. More particularly, the invention relates to such a top guide coupling member which is hydraulically operated to vary its length. The variable length top guide coupling member of the present invention is of the kind having a cylinder carrying a main piston with a piston rod, and articulation means at opposite ends for attachment between an agricultural implement and a farm tractor. Hydraulic lines are connected between a hydraulic system in the tractor and the top guide coupling member to provide a source of hydraulic operating fluid.
Hydraulically operated variable length top guide coupling members have been known in the art, such as from German AS No. 2 025 040, and German No. AS 2 206 195.
In these prior arrangements hydraulic pressure is applied to the piston of the top guide by means of control valves of the slide or piston valve type. Other valves to maintain pressure at a constant level are provided between the slide valves and the cylinder spaces on opposite sides of the piston to prevent leakage losses which inevitably occur in slide valves.
In view of the fact that these constant-pressure valves are set to open at a predetermined pressure, and that this pressure is of the order of magnitude of maximum pressure in the hydraulic system of the tractor, the maximum available working pressure in the cylinders is therefore limited to values which correspond to the pressure in the tractor hydraulic system.
On the other hand, the forces which are likely to occur when working with heavy equipment, e.g., multi-share ploughs, are of such magnitude that they may easily amount to a multiple of the forces which can be controlled by the tractor hydraulic system. In theory this problem might be solved by increasing the cylinder diameter, but this is an impractical solution because of excessively large space requirements that would result.
A further development in hydraulically operated variable length top guide coupling members has been to provide an actual-value donor device in association with a comparator of designed value to actual value and an adjusting device. The control slide will be actuated in such a way that it is possible to set the device for a pre-selected length adjustment and to make sure that this adjusted length will be preserved even under the influence of extraneous forces, by virtue of a continuous comparison between designed and actual values. Such an arrangement also allows a pre-selected length setting to be repeatedly reproduced.
However, the above described control system comparing designed value to actual value is expensive and not very suitable for the rough working conditions to which agricultural equipment is subjected. Pressure limiting valves are provided for the protection of connecting hoses against violent pressure shocks of the kind likely to occur during service. This, however, means a further increase in costs. Additional valves also present an additional source of potential malfunction.
It is therefore the object of the present invention to provide a hydraulically operated variable length top guide coupling member which is capable of absorbing operative pressures considerably in excess of the maximum pressure values in the tractor hydraulic system and which can be manufactured at economically acceptable costs, and wherein the hydraulic lines or hoses will never be exposed to a pressure exceeding the maximum pressure of the tractor hydraulic system.
Other objects, features and advantages of the present invention will become more apparent from the detailed description of the invention in connection with the accompanying drawings, to be described more fully hereinafter.